1. Field of the Invention
The present invention relates to an erasing device for image transfer type copying apparatus, and more particularly to an erasing device for erasing unnecessary charges from a photosensitive member of such apparatus.
2. Prior Art
While there have been proposed various erasing devices such as those described in U.S. Pat. Nos. 3,778,148, 3,746,442 and 3,960,446, each of these devices is either at least structurally or functionally complicated. In the erasing device described in U.S. Pat. No. 3,778,148, a shutter is required to intercept an erasing light which makes the device rather complicated structurally. Similarly, U.S. Pat. No. 3,746,442 requires a light reflective shield which is automatically moved by inertia between the light source and document holder. However, the erasing device described in U.S. Pat. No. 3,960,446 is rather simple structurally but instead requires an erasing lamp and an image exposure lamp to be energized alternatively. However, there is a problem with this device in that the trailing end of the electrostatic latent image formed may become erased by the erasing lamp since the erasing lamp is energized to be lit simultaneously with the extinguishment of the exposure lamp. Thus, there is a need for a novel erasing device free of these drawbacks.
Although the erasing devices described above are primarily concerned with erasing of unnecessary charges resulting from operation of the corona charging means for charging the surface of the photosensitive member prior to image exposure wherein the charging means is continuously operated, there is a copying apparatus such as that shown in U.S. Pat. No. 3,122,634 in which the means for charging the photosensitive member as well as another charging means for effecting transfer of the image onto the copying paper are controlled by a single transformer. The use of a single transformer is particularly effective from the consideration of economy and simplification of electrical control for the copying apparatus, and also effective for improvement of the charging characteristic of short length charging means used in small size copying apparatus capable of producing copies on post cards or small cards.
However, the use of a single transformer to operate both charging means, i.e., means to charge the surface of the photosensitive member and means to effect transfer of the image, will create a problem in its control since one of the charging means cannot be deenergized while the other is continued to be energized. Thus, at least one of these charging means will remain operated even when it is unnecessary, thereby causing more surface area of the photosensitive member to become charged as compared with conventional copying apparatus in which both charging means are controlled by separate transformers. Unless these charges are erased, they will be developed with toner by developing means leading to waste of toner and placing an over-load on the cleaning means. Thus, even if a suitable erasing means is provided to avoid such drawbacks, the operational control thereof would become quite complicated unless some further measures are taken.